Devices and methods for mixing fluids, particularly for rapid mixing of fluids, are employed in many research areas and applications, including the fields of chemistry, e.g. synthetic, analytic and mechanistic research, and in medical/clinical diagnostic procedures. Devices and methods which work on the macroscale accomplish mixing by turbulence, e.g., magnetic stirring bars, electrically powered shakers, and stopped-flow spectroscopy. These devices use moving parts or very high flow rates, for example, to create turbulence, which causes mixing. Devices and methods which work on the microscale, i.e. at low Reynolds number, accomplish mixing by diffusion. At low Reynolds number, e.g. Reynolds number of about one or less, turbulence is negligible and diffusion is the only significant means of mixing. The speed of mixing by diffusion depends on the diffusion coefficients of the particles to be mixed and on the concentration of the particles. In general, the larger the particle and/or the lower the concentration, the longer it will take for mixing to occur.
Devices which use turbulence to effect mixing include static mixers. Static mixers effect mixing by stationary components that deflect substances flowing through a conduit containing the stationary components. For example, European Patent No. EP 0071454 describes a static mixer which employs stationary baffles to deflect the flow of substances through a passage, resulting in mixing of the substances as they flow through the passage. These devices, however, are large and use large volumes of fluids. Because of the baffles or analogous components necessary to effect mixing, it is impossible to form small static mixers which operate at flow speeds in the range of 100 picoliters/second to 10 milliliters/second. They cannot be scaled down to the size of microscale devices which allow for laminar conditions because under laminar flow conditions there is no mixing besides diffusion, i.e. no turbulent mixing occurs.
Microfluidic devices allow one to take advantage of diffusion as a rapid separation mechanism. Flow behavior in microstructures differs significantly from that in the macroscopic world. Due to extremely small inertial forces in such structures, practically all flow in microstructures is laminar. This allows the movement of different layers of fluid and particles next to each other in a channel without any mixing other than diffusion. On the other hand, due to the small widths and depths in such channels, diffusion is a powerful tool to separate molecules and small particles according to their diffusion coefficients, which is usually a function of their size.
Devices which employ diffusion as a means of effecting mixing, in general, have the disadvantage that the rate of mixing is dependent on the rate of diffusion of the substances being mixed and therefore effect mixing at a much slower rate than do devices employing turbulence. Some devices which employ diffusion as a means of mixing are designed to increase the rate of diffusion (and therefore also the rate of mixing) by splitting fluid streams to be mixed into several smaller streams. These smaller streams are then rotated relative to one another, thereby increasing the surface area of contact among the streams and decreasing the distances which the substances must diffuse. The streams are then channeled back together.
PCT publication WO 97/00125 discloses a flow cell for mixing by diffusion which divides each of two or more input streams into a plurality of thin streams and then channels the thin streams into a planar flow bed such that adjacent thin streams which are in contact with each other are from different input streams. Thus, there is an increased surface area of contact between the input streams, a reduced distance for diffusion, and hence a reduced time for mixing under laminar conditions. This device, however, appears to provide for mixing in only one dimension, that is in the plane of the fluid flow, perpendicular to the direction of flow. FIG. 1 shows a generic fluid flow device 1 for the purpose of defining the three axes which represent spatial direction. Fluid flows from the inlet 5 toward the outlet 10. PCT publication WO 97/00125 teaches mixing only in the depth dimension.